1. Field of the Invention
The present invention relates to a solid-state imaging device and a method for making the same, and an imaging apparatus including the solid-state imaging device.
2. Description of the Related Art
Heretofore, various proposals have been made on improvements of sensitivity of solid-state imaging devices. One of such proposals is a waveguide structure in which a hole formed directly above a light receiving unit is filled with a high-refractive-index material (refer to Japanese Unexamined Patent Application Publication Nos. 11-121725, 10-326885, and 2005-294749).
FIG. 5 is a schematic diagram of an example of a solid-state imaging device including a waveguide structure. A light receiving unit (photodiode) 52 is formed in a region isolated by a device isolation layer 53 of a semiconductor substrate 51 such as a silicon substrate. A transfer gate electrode 55 is formed on a gate insulating film 54 on the semiconductor substrate 51 at the left side of the light receiving unit (photodiode) 52. A two-layer wiring layer 58 connected to the semiconductor substrate 51 through a contact layer (conductive plug) 57 is provided. The two-layer wiring layer 58 is covered with an insulating layer 59. A passivation film (protective film) 61, a planarizing film 62, a color filter 63, and an on-chip lens 64 are formed on the insulating layer 59.
A hole formed in the insulating layer 59 above the light receiving unit (photodiode) 52 is filled with a high-refractive-index material layer 60 and constitutes a waveguide. Note that in FIG. 5, an etching stopper film 56 that serves as an etching stopper during formation of the hole in the insulating layer 59 by etching is provided.
The waveguide optically connects the on-chip lens 64 to the light receiving unit (photodiode) 52. Because the high-refractive-index material layer 60 that serves as a core of the waveguide has a refractive index higher than that of the insulating layer 59 serving as a cladding, the incident light having an incident angle greater than the critical angle θ can be totally reflected at the interface between the high-refractive-index material layer 60 and the insulating layer 59. Due to the reflection at the interface, the incident light can be transmitted to the light receiving unit (photodiode) 52 and the efficiency of focusing light on the light receiving unit (photodiode) 52 can be increased.
Japanese Unexamined Patent Application Publication 2005-294749 proposes a structure in which an inner lens having a lens surface at the light receiving unit side is provided above the light receiving unit and the inner lens is formed of a material having a refractive index higher than that of the surrounding layers. According to this structure, the incident light can be focused by the lens surface of the inner lens and guided to the light receiving unit. A hole connected to and provided above the inner lens is filled with a material having a refractive index higher than that of the layers surrounding the hole so as to form a waveguide. As a result, the waveguide can function as a core, the surrounding layers can function as a cladding, and the incident light can be reflected by the side wall of the waveguide and guided to the light receiving unit. Thus, the degree at which incident light is focused on the light receiving unit can be increased and more incident light can be guided into the light receiving unit.